I've had a cat self-destruct. Actually burned itself out and blew chunks into the muffler. (You could hear them rattling.)
After an unscientific review of what might have caused it, I came up with two possibilites.

1.) Air leak in the engine exhaust
2.) Engine had been running rich because of needing a tune and AFM needing replacement.

The interesting thing was that I was expecting to notice a loss in power as I replaced the empty cat with a new one.
I could not tell any difference in power with or without the cat.

Oh, this happened over the period of a year or two as well as I could figure out. (between smog tests)

Todd
'88 Westy

 -------------- Original message ----------------------
From: Jeff Stewart <fonman4277@EARTHLINK.NET>
> OK, despite the cold, wind and snow flurries, I went out and pulled the
> cat-oh yeah, it was plugged. Took my 2 foot section of pipe that I use as a
> breaker bar and a hammer and whacked the guts out of it and reinstalled it.
> Fired up the van, and a fairly large cloud of blueish-white smoke came
> out-I've never seen this engine blow smoke of any color. Took it out for a
> test drive and all was OK for 2 blocks, then stumbling, loss of power and
> backfiring. Turned around to head for home and it started to run OK again.
> So, I kept driving, and now it's intermittent runs fine-looses power,runs
> fine again. I'm beginning to believe my original crappy gas theory.
> Question-could crappy gas have caused the meltdown of the cat? Since I was
> now below half a tank, I took it into town and filled it up with Amoco
> premium. On the 4 mile trip back home, it seemed OK, stumbled a little, but
> was better. I have not swapped out the fuel filter yet, but will do so as
> soon as I  can. Dammit, I should have known better than to buy gas in
> Lynchburg on a Sunday-damn Jerry Falwell!
>
> Jeff Stewart
> fonman4277@earthlink.net
> http://autos.groups.yahoo.com/group/vwcampersmidatlantic/